1. Field of the Invention
The present invention relates to a method for producing an electrolyte for a secondary battery, in particular, a secondary battery having a long cycle life and less performance deterioration due to repeated charge and discharge, the secondary battery utilizing an intercalating reaction and a deintercalating reaction of lithium ions to charge and discharge.
2. Related Background Art
Recently, the amount of CO.sub.2 gas contained in the atmosphere has increased, and it is predicted that the global warming phenomenon would progress due to a greenhouse effect. For this reason, it is difficult to construct a thermal power plant which discharges a large amount of CO.sub.2 gas. Therefore, so-called load leveling is proposed as an effective utilization of electric power generated by a generator in a thermal power plant, while electric power is stored during the night in a secondary battery arranged in a general household, the stored electric power is used in the daytime, when most electric power is used, thereby leveling the load. Further, development of a secondary battery having high energy intensity is essential for electric automobiles that do not discharge air pollution substances. Furthermore, development of a high performance secondary battery that is small in size and low in weight is rapidly demanded as an electric source for portable instruments such as book type personal computers, word processors, video cameras or cellular phones.
Development of a secondary battery using, as a positive electrode material, a lithium intercalation compound which deintercalates lithium ions from interlayers, and metallic lithium as a negative electrode in reactions during charging, or a rocking chair type battery, so-called "lithium ion battery", using, as a negative electrode material, a carbon material represented by graphite which can intercalate lithium ions between interlayers of a 6-membered ring net-like plane formed by carbon atoms, has been considered to achieve a high performance secondary battery having a small size and a low weight (in the present invention, a secondary battery utilizing intercalation and deintercalation reactions of lithium ions due to charge and discharge is hereinafter called a lithium secondary battery, including a "lithium ion battery" using carbon material as a negative electrode).
However, a high capacity lithium secondary battery using metallic lithium as a negative electrode has a very short cycle life of charge and discharge, and does not yet reach a practical level. According to the investigations by the present inventors, the main cause that a cycle life of charge and discharge of a lithium secondary battery using metallic lithium as a negative electrode is very short is presumed as follows.
"Metallic lithium is reacted with impurities such as water or an organic solvent in an electrolytic solution to form an insulating film, and due to this, lithium metal grows in a dendrite (tree branch) shape by repeated charge and discharge, thereby inducing internal short circuit between a negative electrode and a positive electrode; alternatively a dissociated electrolyte is converted to a polymerization initiator, so that an organic solvent in an electrolytic solution is polymerized to increase impedance inside a battery, and decomposition of an electrolytic solution is promoted by repeated charge and discharge, thus reaching its life".
Further, where the dendrite of the lithium described above grows and a negative electrode and a positive electrode form a short circuit, energy of a battery may be consumed by the short circuit in a short period of time, thereby generating heat, or a solvent of an electrolytic solution may be decomposed to generate a gas, thereby increasing inner pressure, leading to the breakage of a battery.
On the other hand, in the above-described "lithium ion battery", a negative electrode constituted of a carbon material of graphite structure theoretically intercalates only lithium atom of at most 1/6+L per carbon atom. Therefore, a secondary battery having high energy intensity comparable to a lithium primary battery when metallic lithium is used as a negative electrode material is not yet realized. If it is attempted to intercalate lithium on a carbon material negative electrode of the "lithium ion battery" in an amount exceeding the theoretical amount, in charging, lithium metal precipitates on the surface of the carbon material as a negative electrode. Similar to the case of the above-described battery using lithium metal as a negative electrode, the metallic lithium reacts with impurities such as water, or an organic solvent in an electrolytic solution to form an insulating film, and the metallic lithium grows in a dendrite (tree branch) shape by repeated charge and discharge to induce inner short circuit between a negative electrode and a positive electrode, reaching its life. Thus, practically sufficient cycle life is not yet obtained in the "lithium ion battery" that exceeds the theoretical capacity of a graphite negative electrode.
Therefore, increase in energy intensity and prolongation of cycle life are strongly demanded in a lithium secondary battery.
To achieve this demand, Japanese Patent Application Laid-Open No. 9-82359 discloses an electrolyte comprising a salt of an organofluorosilicon compound in a lithium secondary battery to reduce deterioration in performance due to repeated charge and discharge, thereby prolonging the cycle life.